Categories page is located at yoursite.mirocommunity.org/admin/categories.
To access it:
1. Select "Videos" item from the main menu on the left side of the administration interface.
2. Click on "Categories" tab.
This page allows administrators to add, delete or edit categories.
Although the Categories page will automatically update with new categories, it may take some time for newly added or deleted categories to appear properly in the list of categories elsewhere on the site.
You can read more about Categories in the How-To article: Adding Categories.